Most common defects in human vision are caused by the shape of the cornea of the eye. For example, nearsightedness can be attributed to a cornea in which the surface curvature is too small, farsightedness can be attributed to a cornea in which the surface curvature is excessive, and astigmatism can attributed to a cornea with irregular surface curvature. Ophthalmologists model the cornea as a portion of an ellipsoid defined by orthogonal major and minor axes. Surgical procedures for correcting visual acuity are typically directed at increasing or decreasing the surface curvature of the cornea, or making its shape more spherical.
The human cornea is an “asymmetrically aspheric” surface. “Aspheric” on the one hand means that the radius of curvature along a corneal “meridian” (which is an imaginary line on the corneal surface passing through the geometric center of the cornea, analogous to a geographic meridian) is not a constant. Indeed, the corneal curvature flattens progressively from the geometric center to the periphery. “Asymmetric” on the other hand means that the profile of the corneal curvature along a half-meridian is not the same as (i.e., it is not a mirror image of) the other half of the same meridian. The degree to which corneas are aspheric and/or asymmetrical varies from patient to patient.
In conjunction with modern corneal procedures, such as radial keratotomy and corneal ablation surgery, and for clinical applications, high resolution cameras are used to obtain a digitized array of discrete data points on the corneal surface. One system and camera useful for mapping the cornea is the PAR Corneal Topography System (PAR CTS) available from PAR Vision Systems. The PAR CTS maps the corneal surface topology in two-dimensional Cartesian space, i.e., along x- and y-coordinates, and locates the “line-of-sight”, which is then used by the practitioner to plan the surgical procedure. The “line-of-sight” is a straight line segment from a fixation point to the center of the entrance pupil. As described more fully in Mandell, “Locating the Corneal Sighting Center From Videokeratography,” J. Refractive Surgery, V. 11, pp. 253-259 (July/August 1995), a light ray which is directed toward a point on the entrance pupil from a point of fixation will be refracted by the cornea and aqueous and pass through a corresponding point on the real pupil to eventually reach the retina.
The point on the cornea at which the line-of-sight intersects the corneal surface is the “optical center” or “sighting center” of the cornea. It is the primary reference point for refractive surgery in that it usually represents the center of the area to be ablated in photorefractive keratectomy and the center of the area to be spared in radial keratotomy. The line-of-sight has conventionally been programmed into a laser control system to govern corneal ablation surgery and as a reference line about which a radial keratotomy procedure is performed. However, some surgeons prefer to use the pupillary axis as a reference line. Experienced practitioners have employed various techniques for locating the sighting center. In one technique, the angle lambda is used to calculate the position of the sighting center relative to the pupillary (“optic”) axis. See Mandell, supra, which includes a detailed discussion of the angles kappa and lambda, the disclosure of which is incorporated herein by reference as if set forth in its entirety herein.
In corneal ablation procedures, a portion of the corneal surface is ablated. The gathered elevational data is used to direct an ablation device, such as a laser, so that the corneal surface can be selectively ablated to more closely approximate a spherical surface of appropriate radius about the line-of-sight, within the ablation zone. In radial keratotomy (“RK”) procedures, a series of radially directed, circumferentially spaced incisions are made on the corneal surface about the line-of-sight to weaken the corneal walls and permit the cornea to sit flatter, that is, to have a larger radius of curvature.
In ablation and RK procedures, the use of the line-of-sight as a reference line for the procedures may reduce myopia or otherwise correct a pre-surgical dysfunction. However, a more irregularly shaped cornea may result, which may exacerbate existing astigmatism or introduce astigmatism in the treated eye. For example, an RK procedure that is performed with respect to the line-of-sight typically reduces myopia, but the incisions, which are made of equal length when viewed in two-dimensional projection, are actually of unequal length along the surface of the cornea, unless the cornea has a symmetrical topology. The RK procedure will therefore introduce an undesirable asymmetry in the cornea, resulting in irregular astigmatism. This will complicate any subsequent vision correction measures that need be taken. Also, any substantial surface irregularities which are produced can cause development of scar tissue or the local accumulation of tear deposits, either of which can adversely affect vision.
Implicit in the use of the-line-of sight or the pupillary axis as a reference axis for surgical procedures is the assumption that the cornea is symmetric about an axis extending along a radius of the eye. However, clinical measurements performed with the PAR CTS, as analyzed in accordance with the method of the present invention, reveal that the cornea exhibits a tilt, typically a forward and downward tilt, relative to the eye. This tilt may be as great as 6° and, on the average, is between 2° and 3°. Hence, a corneal ablation procedure which utilizes the line-of-sight or pupillary axis as a reference axis tends to over-ablate some portions of the cornea and underablate other portions of the cornea. At the same time, it changes the geometric relationship between the cornea and the remainder of the eye. Thus, any ablation procedure which does not take into account the tilt of the cornea is not likely to achieve the desired shaping of the cornea and may therefore be unpredictable in its effect.
Analysis of clinical measurements in accordance with the method of the present invention also reveals that that point on the surface of the cornea which is most distant from the reference plane of the PAR CTS (hereafter referred to as the HIGH point) is a far more effective reference point for corneal ablation than the center of the cornea. Specifically, as demonstrated below, laser ablation about an axis passing through the HIGH point produces a much more regularly shaped cornea and removes substantially less corneal material than the same operation performed about an axis close to the center of the eye, such as the pupillary axis.
Recently, the use of a collagen gel has been proposed as a vehicle to facilitate smoothing of the corneal undulations. See Ophthalmology Times, “Slick Start, Clear Finish,”1995, pp. 1 and 24 (Jun. 19-25, 1995) and Review of Ophthalmology, “News & Trends: Researchers Unveil New Ablatable Mask” pp. 12-13 (June 1995), the disclosures of which are incorporated herein by reference as if set forth in their entirety herein. A Type 1 collagen is molded between a contact lens and the anterior surface of the cornea to form a gel mask. The surgeon can adjust the curvature of the postoperative cornea by selecting a flatter or steeper lens, as desired. Reportedly, the gel mask does not shift when hit by laser pulses. Therefore, instead of selective ablation of predetermined locations of the cornea, the masked cornea can be ablated to a uniform depth, thereby conforming the surface contour of the cornea to the lens. A smooth post-operative cornea results, and refractive power correction can be achieved. However, because the ablation operation is centered on the optical center of the cornea or the center of the pupil and does not allow for corneal tilt, the postoperative eye may exhibit an irregular shape or more corneal material may be removed than is necessary.
What is needed in the art and has heretofore not been provided is a method of correcting vision that avoids one or more of these problems, that can produce predictable results, and that provides corrected vision with respect to the particular topology of the patient's eye on which the correction is being performed.
It is therefore one object of the present invention to provide a method for improving the vision of an eye.
It is an additional object of the present invention to provide an improved surgical method for a corneal ablation procedure.
It is a further object of the invention to provide an improved surgical method for an RK procedure.
It is also an object of the present invention to provide a method and apparatus for diagnosing and analyzing a pre-surgical eye for the purpose of predicting the post-operative condition of the eye and planning more effective surgery.
In accordance with the invention, these objects are achieved by causing the optical center of the eye to align with the “HIGH point” of the cornea.
In accordance with the present invention, these objects are achieved, in part, by performing corneal ablation and RK procedures of the eye in a manner which does not interfere with the natural tilt of the cornea relative to the remainder of the eye.
According to one aspect of the invention relating to corneal ablation procedures, the HIGH point of the cornea is used as the pole of a spherical surface which is fitted approximately to a portion of the anterior surface of the cornea within a “bounded region.” For corneal ablation procedures, the “bounded region” comprises a generally inverted-cup shaped region of the anterior surface of the eye bounded at its periphery by a plane which is substantially perpendicular to the z-axis of the bounded region. During the operation local high points which project above the spherical surface are ablated.
According to another aspect of the invention, ablation surgery of the eye is performed by first depositing a settable collagen gel on the cornea, placing a lens on the collagen so that the center of the lens aligns with the HIGH point of the cornea, molds the collagen between the lens and cornea, and permits the collagen gel to set whereby the anterior surface is formed into a spherical mask which is centered over the HIGH point. The mask is then ablated to a uniform depth.
According to another aspect of the invention relating to radial keratotomy procedures, a pair of incisions in the plane of a “great circle” is formed in the cornea to weaken and flatten it. As used herein, a “great circle” is formed by a plane containing the HIGH point and parallel to the z-axis of the bounded region. The “bounded region” for radial keratotomy procedures is defined absolutely in terms of a circle projected onto the corneal surface which is centered about an axis passing through the HIGH point and parallel to the z-axis of the bounded region.
According to a further aspect of the invention, an apparatus for performing radial keratotomy on an eye is disclosed. The apparatus preferably includes a housing adapted to be selectively attached to the cornea by a vacuum so that the housing remains in a fixed position with respect to the cornea during radial keratotomy. A first means associated with the housing is provides a first, regular shape on the cornea which is circular in two-space. A second means associated with the housing provides a second, irregular shape on the cornea. The first and second shapes are disposed on the anterior surface of the cornea such that one surrounds the other. The apparatus further includes a radially movable blade mounted so that radial movement of the blade in the plane of the great circle from one of the first and second shapes to the other on either side of the HIGH point of the cornea results in incisions in the cornea.
According to yet another aspect of the invention, vision correction is achieved without surgery. Use is made of a contact lens of a type which assumes a predetermined position and orientation when placed in a patient's eye, and the lens is constructed so that its optical center is aligned with the HIGH point of the cornea when the lens is inserted into the patient's eye. Thus when the lens is worn, the optical center of the eye and HIGH point of the cornea are aligned.
These and other objects, features and advantages of the present invention will be readily apparent from the following detailed description taken in conjunction with the accompanying unscaled drawings.